Dazzling as it often seems, fashion in the animal kingdom can be frighteningly repetitive. There are so many color patterns that scream “look at me” among the grays and greens of foliage and mud.
So it should come as no surprise that animals often use the same colors for very different purposes.
The brilliant crimson of a male northern cardinal (Cardinalis cardinalis) serves as a signal for potential mates to approach. in frogs strawberry poison darts (Oophaga pumilio), this burst of red is a stern warning to keep away or you’ll swallow a mouthful of potent, deadly toxin.
Evolutionary biologist Zachary Emberts, now of Oklahoma State University, and colleague John Wiens of the University of Arizona, wondered what makes the same colors evolve to serve such different purposes in different animals.
They conducted a study of 1,824 species of land vertebrates (aquatic animals can be another kettle of fish), categorizing their coloration as either coming-here or disappearing, and found the common thread connecting each group.
The animals that come here, like birds and lizards, are descended from ancestors that were diurnal or active during the day. Animals that go extinct, such as snakes and amphibians, are descended from nocturnal ancestors.
“The traits we see in species today may be the result of their evolutionary history,” says Emberts. “We were looking for evolutionary patterns, so we did two separate analyses, one that used their current day-night activity and one that used their ancestral day-night activity.”
There is no correlation, they found, between day and night activity and animal coloration today. Rather, the link is purely ancestral. But it’s something that seems to be consistent across all land vertebrates, whose evolution goes back about 350 million years.
“It doesn’t matter how a species produces the colors,” says Wiens. “The way a bird turns red is different than how a lizard turns red, but this general pattern of day-night activity still works.”
According to the researchers’ analysis, most of the ancestors of the animals they studied started out rather plain and monochromatic, evolving their vivid hues over time, and most of them live in environments where their vivid colors stand out. The most logical explanation is that more brightly colored animals were better able to survive and pass their genetic material on to generations that continued the trend.
The colors analyzed included red, orange, yellow, purple and blue, and the researchers found that, for all colors except blue, colorations were equally divided between sexual signals and warning. It is currently unclear what could be the reason for this.
“It’s interesting to see that for certain colors like red, orange and yellow, they are used with similar frequency both as a way to avoid predators and as a way to attract a mate,” says Emberts.
“On the other hand, blue coloration was more often associated with mating as opposed to predator avoidance.”
The coloration of diurnal animals makes sense: a showy animal, in daylight, is going to be seen by other animals, including potential mates. This may make them bigger targets for predators as well, but it seems that being able to mate and reproduce is more important than not being eaten. Females of these species are often monotonous in comparison, and are therefore better able to hide from predators and survive to rear offspring.
But nocturnal animals slip and spy in the dark. A male nocturnal snake does not have much use for a bright color for sexual signaling if females cannot see it.
“Warning colors have evolved even in eyeless species,” says Wiens. “It is doubtful that most snakes or amphibians can see color, so their bright colors are generally used to signal to predators rather than members of the same species.”
Instead, the researchers suggest, the coloration may have evolved as a way to tell diurnal predators what might happen to the sleeping animal to move away. But future research may reveal more details. The team hopes to delve into the evolution of the bright colors to see if their functions have changed over time.
Meanwhile, however, research shows that delving into the evolutionary history of animal traits can reveal patterns that are no longer relevant today.
The team’s research was published in Development.
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